See also Good Is Not Soft, when a normally nice character realizes that nice will not get things done in a situation. They can coincide, if the realization and the outrage are triggered at the same time. The results are quite similar if they Madden Into Misanthropy, the difference being the new misanthrope isn't so much a violent dynamo as a care free Jerkass.

This is not to be confused with a character being a Bitch in Sheep's Clothing. While the latter trope's 'nice' image is usually just a facade for their genuinely callous personality, Beware the Nice Ones involves a genuinely nice character submitting to a rare act of malice. Repeated subjection of a nice character to this trope, though, may result in Flanderizing them to such a degree that it's impossible to tell the difference.